FIG. 21 is a front view illustrating an example of a known ultrasonic sensor, and FIG. 22 is a plan view of the ultrasonic sensor taken along line A-A in FIG. 21. An ultrasonic sensor 1 includes a case 2 composed of a metal such as aluminum having a substantially rectangular tube shape with a bottom portion. The case 2 has a recessed portion on the inside and a rectangular or an elliptic opening so that the ultrasonic sensor 1 achieves a desired directivity. A first surface of a-piezoelectric element 3 is bonded to the inner surface of the bottom portion of the case 2 using an adhesive. Moreover, a first signal line 4a of a cable 4 is soldered to the inner surface of the case 2, and is electrically connected to an electrode on the first surface of the piezoelectric element 3 via the case 2. Furthermore, a second signal line 4b of the cable 4 is soldered to an electrode on a second surface of the piezoelectric element 3 so as to be electrically connected. Moreover, a sound absorber 5 composed of, for example, felt is disposed on the second surface of the piezoelectric element 3. Furthermore, an insulating resin filler 6 composed of, for example, silicon rubber or urethane rubber is disposed inside the case 2. This insulating resin filler 6 hermetically seals the piezoelectric element 3 and the sound absorber 5, and insulates the signal lines 4a and 4b from each other.
When a distance to an object to be detected is measured using this ultrasonic sensor 1, the piezoelectric element 3 is driven by applying driving voltages to the signal lines 4a and 4b of the cable 4. The bottom surface of the case 2 also vibrates in response to the vibration of the piezoelectric element 3, and emits ultrasonic waves in a direction orthogonal to the bottom surface. When the ultrasonic waves emitted from the ultrasonic sensor 1 are reflected from the object to be detected and reach the ultrasonic sensor 1, the piezoelectric element 3 vibrates. The vibration is converted into electrical signals, and the electrical signals are output from the signal lines 4a and 4b of the cable 4. Therefore, the distance from the ultrasonic sensor 1 to the object to be detected can be measured by measuring the time period from the application of the driving voltages to the output of the electrical signals (see Patent Document 1).
Herein, the resonant frequency of this ultrasonic sensor 1 is 40 kHz. FIG. 23 illustrates the reverberation characteristic of the ultrasonic sensor 1 when driving voltages are applied.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-284896
The ultrasonic sensor 1 shown in FIG. 21 has the following problems.
First, since the piezoelectric element 3 is bonded to the surface of the bottom portion of the case 2 using an adhesive, the amount or the bonding position of the adhesive may vary, resulting in variations in ultrasonic-wave transmitting/receiving characteristics.
Moreover, it is necessary to form the metallic case 2 with a complicated shape so that a desired directivity is achieved, and the transmitting/receiving characteristics may vary in accordance with the variations among the manufactured cases 2.
Furthermore, since the case 2 is composed of a metal, the case 2 may be oxidized at the portion where the piezoelectric element 3 is bonded. This also leads to variations in the ultrasonic-wave transmitting/receiving characteristics.